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United States Patent |
6,115,005
|
Goldstein
,   et al.
|
September 5, 2000
|
Gain-optimized lightweight helical antenna arrangement
Abstract
A gain-optimized, compact helical antenna array comprises an array of
tapered pitch angle helical antenna elements. By tapered pitch angle is
meant that the pitch angle increases from the base end of the antenna
element to the distal end, in a manner that optimizes the gain of each
helical element relative to helix length for a given physical size of the
winding. Each helical winding is coupled to a signal distribution network,
through which the antenna's radiation pattern is controllably defined. The
antenna elements have a spatially aperiodic distribution, that reduces
grating lobes, by minimizing the number of antenna elements which share
the same azimuth. A radial line orthogonal to the boresight axis will
intercept a minimum number of helical antenna elements of the array. To
minimize mutual coupling, the mutual spacing between any two antenna
elements of the array is at least a prescribed minimum separation that is
proportional to a product of the square root of the gain of the respective
antenna element and the wavelength of the operating frequency of the
array.
Inventors:
|
Goldstein; M. Lawrence (Palm Bay, FL);
Gyorko; Eric Andrew (Indialantic, FL);
Killen; William D. (Satellite Beach, FL)
|
Assignee:
|
Harris Corporation (Melbourne, FL)
|
Appl. No.:
|
106433 |
Filed:
|
June 29, 1998 |
Current U.S. Class: |
343/895 |
Intern'l Class: |
H01Q 001/36 |
Field of Search: |
343/895,844,850
|
References Cited
U.S. Patent Documents
5345248 | Sep., 1994 | Hwang et al. | 343/895.
|
5453755 | Sep., 1995 | Nakano et al. | 343/895.
|
5528771 | Nov., 1993 | Praba | 343/895.
|
5541617 | Jul., 1996 | Connolly et al. | 343/895.
|
5892480 | Apr., 1999 | Killen | 343/895.
|
Primary Examiner: Wong; Don
Assistant Examiner: Nguyen; Hoang
Attorney, Agent or Firm: Wands; Charles E.
Claims
What is claimed:
1. An antenna arrangement comprising:
a plurality of helical antenna elements extending from a generally planar
surface, each helical antenna element having a helical antenna axis
orthogonal to said generally planar surface, and being distributed around
a boresight axis of said antenna arrangement, said boresight axis being
orthogonal to said generally planar surface such that any radial line
through and orthogonal to said boresight axis and intercepting a helical
axis of any of said helical antenna elements intercepts a helical antenna
axis of no more than one other of said helical antenna elements; and
a power distribution circuit configured to interface a signal input/output
port with said plurality of helical antenna elements.
2. An antenna arrangement according to claim 1, wherein each of said
helical antenna elements is spaced apart from every other helical antenna
element of said plurality by at least two wavelengths of an operating
frequency of said antenna.
3. An antenna arrangement according to claim 2, wherein mutual spacing
between any two helical antenna elements of the array conforms with a
prescribed gain-based spacing relationship, such that any helical antenna
element is spaced apart from any other helical antenna element by a
minimum separation that is proportional to a product of the square root of
the gain of the respective helical antenna element and the wavelength of
the operating frequency of said antenna arrangement.
4. An antenna arrangement according to claim 3, wherein a respective
helical antenna element i is spaced apart from every other helical antenna
element j by a minimum spacing Sij=(Gi/4pi).sup.1/2 *lambda, where Gi is
the gain of said respective helical antenna i, and lambda is the
wavelength of the operating frequency of said antenna arrangement.
5. An antenna arrangement according to claim 3, wherein said helical
antenna elements comprise variable pitch helical antenna elements.
6. An antenna arrangement according to claim 5, wherein the pitch angle
.alpha..sub.i in degrees of a respective winding wi of a respective one of
said plurality of helical antenna elements, relative to a feed location
thereof, is equal to 5+(i-1)(N-1).sup.-1 (10logN-5) degrees, where N is
the total number of turns of said respective winding.
7. An antenna arrangement according to claim 6, wherein a respective
helical antenna element i is spaced apart from every other helical antenna
element j by a minimum spacing Sij=(Gi/4pi).sup.1/2 *lambda, where Gi is
the gain of said respective helical antenna i, and lambda is the
wavelength of the operating frequency of said antenna arrangement.
8. An antenna arrangement according to claim 1, wherein said helical
antenna elements comprise variable pitch helical antenna elements.
9. An antenna arrangement according to claim 8, wherein the pitch angle
.alpha..sub.i in degrees of a respective winding wi of a respective one of
said plurality of helical antenna elements, relative to a feed location
thereof, is equal to 5+(i-1)(N-1).sup.-1 (10logN-5) degrees, where N is
the total number of turns of said respective winding.
10. A gain-optimized, compact helical antenna array comprising a spatially
aperiodic array of tapered pitch angle helical antenna elements extending
from a generally planar surface, each helical antenna element having a
helical antenna axis orthogonal to said generally planar surface, and a
signal distribution network, to which each helical antenna element is
coupled and through which the antenna's radiation pattern is controllably
defined, and wherein said spatially aperiodic distribution is such that
for any helical antenna element, a radial line orthogonal to a boresight
axis of said array, said boresight axis being orthogonal to said generally
planar surface, and intercepting a helical axis of any of said helical
antenna elements intercepts a helical antenna axis of no more than one
other of said helical antenna elements of the array, and wherein mutual
spacing between any two antenna elements of the array is at least a
minimum spacing Sij=(Gi/4pi).sup.1/2 *lambda, where Gi is the gain of a
respective helical antenna element i, and lambda is the wavelength of the
operating frequency of said array.
11. A gain-optimized, compact helical antenna array according to claim 10,
wherein the pitch angle .alpha..sub.i in degrees of a respective winding
wi of a respective one of said plurality of tapered pitch helical antenna
elements, relative to a feed location thereof, is equal to
5+(i-1)(N-1).sup.-1 (10logN-5) degrees, where N is the total number of
turns of said respective winding.
12. An antenna arrangement comprising:
a plurality of variable pitch helical antenna elements extending from a
generally planar surface, each helical antenna element having a helical
axis orthogonal to said generally planar surface, and being arranged in a
spatially aperiodic distribution relative to a boresight axis of said
antenna arrangement, said boresight axis being orthogonal to said
generally planar surface; and
a power distribution circuit configured to interface a signal input/output
port with said plurality of variable pitch helical antenna elements, and
wherein
said plurality of helical antenna elements are parallel to and distributed
around said boresight axis and intercepting a helical axis of any of said
helical antenna elements of said antenna arrangement, such that a radial
line through and orthogonal to said boresight axis intercepts a helical
antenna axis of no more than one other of said helical antenna elements.
13. An antenna arrangement according to claim 12, wherein each of said
helical antenna elements is spaced apart from every other helical antenna
element of said plurality by at least a minimum spacing of
Sij=(Gi/4pi).sup.1/2 *lambda, where Gi is the gain of a respective helical
antenna element i, and lambda is the wavelength of the operating frequency
of said antenna arrangement.
14. An antenna arrangement according to claim 12, wherein mutual spacing
between any two helical antenna elements of said array conforms with a
prescribed gain-based spacing relationship, such that any helical antenna
element is spaced apart from any other helical antenna element by a
minimum separation that is proportional to a product of the square root of
the gain of the respective helical antenna element and the wavelength of
the operating frequency of said antenna arrangement.
15. An antenna arrangement according to claim 14, wherein a respective
helical antenna element i is spaced apart from every other helical antenna
element j by a minimum spacing Sij=(Gi/4pi).sup.1/2 *lambda, where Gi is
the gain of said respective helical antenna i, and lambda is the
wavelength of the operating frequency of said antenna arrangement.
16. An antenna arrangement according to claim 12, wherein the pitch angle
.alpha..sub.i in degrees of a respective winding wi of a respective one of
said plurality of variable pitch helical antenna elements, relative to a
feed location thereof, is equal to 5+(i-1)(N-1).sup.-1 (10logN-5) degrees,
where N is the total number of turns of said respective winding.
Description
FIELD OF THE INVENTION
The present invention relates in general to communication systems, and is
particularly directed to a new and improved compact array of high gain,
axial mode helical antenna elements, distributed aperiodically around a
boresight axis, in a prescribed spatial geometry and mutual separation
that are effective to minimize grating lobes and optimize the gain of the
antenna array.
BACKGROUND OF THE INVENTION
Because of the substantial size and weight penalties, plus aperture
blockage, associated with the use of parabolic reflector antennas,
communication system users are increasingly turning to reduced mass
antenna arrays for high gain applications, such as, but not restricted to
power-limited satellite communication terminals. A typical high gain axial
mode helical antenna array may employ a plurality of spaced apart
identical antenna elements, the gain of each of which is less than that of
the array. Each of the elements of the array is summed by a shared signal
distribution network, through which the antenna beam or radiation pattern
may be controllably steered or scanned relative to the antenna's boresight
axis.
A shortcoming of an antenna array of high gain antenna elements spaced
sufficiently far apart to avoid mutual coupling is the presence of grating
lobes of substantial magnitude, which perturb the desired sensing
direction of the antenna's main beam. In addition, since grating lobes
represent directivity in unwanted directions, they may reduce the
effective antenna gain in the desired direction. Unfortunately, if the
array elements are placed relatively close to one another, in an effort to
reduce or eliminate the grating lobe problem, mutual coupling effects
between the more closely spaced elements will modify the radiation
patterns of the antenna elements; in an unpredictable manner, thereby
distorting the composite pattern of the array.
SUMMARY OF THE INVENTION
In accordance with the present invention, these mutual coupling and grating
lobe problems: of conventional high gain axial mode helical antenna
arrays, described above, are substantially reduced or eliminated by means
of a relatively compact helical antenna configuration, in which a
plurality of reduced length, tapered pitch helical antenna elements are
distributed aperiodically around the antenna's boresight axis, in a
prescribed geometry and mutual separation that are effective to minimize
grating lobes and optimize the gain of the overall array.
For this purpose, the gain-optimized, helical antenna architecture of the
present invention comprises an array of variable or tapered pitch angle
helical antenna elements mounted to one side of a support structure, that
is affixed to an associated mounting and pointing arrangement. By tapered
pitch angle is meant that the pitch angle successively increases from the
base end of the antenna element to the distal end, in a manner that
optimizes the gain of each helical element relative to helix length for a
given physical size of the winding. At its base end, each helical winding
is coupled to a coaxial feed-through element which passes through the
support base for connection to a section of signal coupling cable from a
signal distribution network, through which the antenna's radiation pattern
may be controllably defined.
Pursuant to the invention, the antenna elements have a spatially aperiodic
distribution, that serves to minimize grating lobes, by minimizing the
number of antenna elements which share the same azimuth. This is
effectively achieved by arranging the antenna elements, such that, for any
given antenna element of the array, a radial line orthogonal to the
boresight axis will intercept a minimum number of helical antenna elements
of the array.
In addition, in order to minimize mutual coupling between elements, the
mutual spacing between any two antenna elements of the array is at least
twice the wavelength of the operating frequency of the antenna for a 17
dBiC helix. The mutual spacing between any two elements of the array
conforms with a prescribed gain-based spacing relationship, such that any
antenna element is spaced apart from any other antenna element by a
minimum separation that is proportional to a product of the square root of
the gain of the respective helical antenna element and the wavelength of
the operating frequency of the array.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic front end or boresight view of a gain-optimized,
helical antenna architecture in accordance with the present invention;
FIG. 2 is a diagrammatic side view of the gain-optimized, helical antenna
architecture of FIG. 1;
FIG. 3 is a diagrammatic rear or feed side view of the gain-optimized,
helical antenna architecture of FIG. 1;
FIG. 4 diagrammatically illustrates the manner in which the pitch angle of
an individual helical antenna element winding of the architecture of FIGS.
1-3 is tapered in accordance with the number of turns of the winding; and
FIG. 5 is effective irradiated power (EIRP) density vs. scan angle
characteristic of a gain-optimized, helical antenna architecture of the
present invention.
DETAILED DESCRIPTION
A preferred embodiment of the gain-optimized, helical antenna architecture
of the present invention is diagrammatically illustrated in FIGS. 1-3, as
comprising an array of axial mode helical antenna elements 10, which are
mounted to a first side 21 of support structure, shown as a generally
planar, circular support base 20. As a non-limiting example, the
embodiment of FIGS. 1-3 shows a thirty-six element array. The support base
20 may comprise a pair of conductive (e.g., aluminum) plates that are
spaced apart by and laminated with an intermediate honeycomb lattice,
thereby providing a relatively lightweight structure, facilitating
deployment in a variety of terminal environments. The support base 20 is
affixed to an associated mounting and pointing arrangement, not shown, as
by means of a pair of gimbal plates 23 at a diametrically opposed
locations at the perimeter of the support base 20.
Each axial mode helical antenna element 10 preferably has a tapered winding
configuration of the type described in copending U.S. patent application
Ser. No. 08/838,546, by William D. Killen, entitled: "Variable Pitch Angle
Axial Mode Helical Antenna," filed Apr. 9, 1997, assigned to the assignee
of the present application and the disclosure of which is incorporated
herein. As such, each helical antenna includes a generally cylindrical
rectilinear shaft 11, a base portion 12 of which is mounted by way of a
circular mounting bracket 13 to the first side 21 of the support base 20,
so that the helical antenna element extends normal to the first side 21 of
the support plate and is parallel to the antenna's boresight axis 30.
Extending from the distal end 14 of each helical antenna element 10 is a
helical winding 15, the pitch of which tapers from a maximum pitch at the
distal end 14 of the element to a minimum pitch at its base portion 12, as
will be described below with reference to FIG. 4.
Each axial mode helical winding 15 is coupled to a coaxial feed-through
element, which passes through the support base 20 to the second side 22
thereof, for connection to a first end 41 of a section of signal coupling
cable 40, a second end 43 of which is coupled to a signal distribution
unit 50, that is mounted to the second side 22 of the support plate. The
signal distribution unit 50 contains a signal network through which the
antenna beam or radiation pattern may be controllably defined relative to
the boresight axis 30.
As pointed out briefly above, pursuant to the invention, the axial mode
helical antenna elements 10 have a tapered pitch configuration and are
distributed around the antenna boresight axis 30, in a prescribed
spatially aperiodic array having an irregular distribution geometry and
mutual separation that are effective to minimize grating lobes and
optimize the gain of the overall array. In particular, in accordance with
a first aspect of the invention, the antenna elements have a spatially
aperiodic distribution, that serves to minimize grating lobes by
minimizing the number of antenna elements which share the same azimuth.
This is effectively achieved by arranging the elements, such that, for any
given antenna element of the array, a radial line orthogonal to the
boresight axis will intercept a minimum number of (e.g., only one or two)
helical antenna elements of the array. In other words, no matter what the
azimuth look angle (relative to the center of the array), that look angle
will intercept or be aligned with a minimum number of helical elements of
the array.
Such a spatially aperiodic relationship is diagrammatically illustrated in
the distribution of FIG. 1, wherein a radial line 25i passing through an
arbitrary antenna element 10i of the array, and being orthogonal to the
boresight axis 30 (e.g., lying in the plane of the first side 21 of the
support base 20) intercepts no more than one other helical antenna element
of the array, here only antenna element 10j.
In accordance with a second aspect of the invention, to minimize mutual
coupling between elements, the mutual spacing among the elements of the
array should be as large as practical. (For the case of a relatively
simple example of only three elements in the array , this means that the
elements would be distributed around the boresight axis at successive
120.degree. mutual angular separations and maximum separation allowed by
the mounting structure). In practical terms (e.g., for a relatively large
number of elements, such as the thirty-six element array shown in FIG. 1,
for example), the spacing between any two antenna elements of the array is
defined as being at least twice the wavelength of the operating frequency
of the antenna, for a 17 dBiC helix.
Specifically, the mutual spacing between any two elements of the array
conforms with a prescribed gain-based spacing relationship, wherein a
first arbitrary antenna element 10i is spaced apart from any other antenna
element 10k by a minimum spacing Sik=(Gi/4pi).sup.1/2 *lambda, where Gi is
the gain of the respective helical antenna 10i, and lambda is the
wavelength of the operating frequency of the array.
Pursuant to a third aspect of the invention, in order to maximize the
composite gain characteristic of the array in a relatively compact
architecture, the gain of each helical element is optimized relative to
helix length for a given physical size of the winding, by tapering the
pitch along each helical winding, as described for example, in the
above-referenced Killen application. To maximize the gain/length ratio for
a given size of a respective helical element, the taper of the helical
winding pitch is defined in accordance with the number of turns of the
helical winding.
This relationship is such that, at any location along its length, the
tapered winding of a respective axial mode helical antenna element has a
pitch angle that is tailored to optimize the exchange of energy between a
free space wave and current flowing in the helical winding. This pitch
angle relationship is diagrammatically illustrated in FIG. 4, wherein the
pitch angle is the angle .alpha. between a plane 43 normal to the winding
axis 40 and a line 44 tangential to the selected location 45 on the
helical winding. The largest value of pitch angle .alpha. is at the distal
end 14 of the antenna shown in FIG. 2, while the smallest value of pitch
angle .alpha. is at the feed port at its base 12. For C-band operation,
the pitch angle .alpha. at the distal end of the antenna, which the
spacing between turns is largest, may have a value on the order of 20-30
degrees (and particularly on the order of 23-26 degrees), while the pitch
angle .alpha. at the feed port 36, where the spacing between turns is
smallest, may have a value on the order of 3-8 degrees (and particularly
on the order of 3-6 degrees).
Between these distal and feed locations, the pitch angle along successive
turns of the helical winding 15 varies in accordance with the relationship
between the phase velocity of a wave propagating through the antenna and
the phase velocity of a free space electromagnetic wave interfaced with
the antenna. Parametric measurements along successive turns of the antenna
have revealed that this phase velocity variation is not linear. As a
consequence, it is preferred that the pitch angles of successive turns of
the antenna be varied in a corresponding non-linear manner, so as to
optimally match the phase velocity of a free space electromagnetic wave
interfaced with (received or launched by) the antenna with the phase
velocity of the wave travelling through the antenna. What results is an
axial mode, helical antenna that has several more dB of gain than would
otherwise be provided by a constant pitch angle configuration of similar
axial length. Also, the variable pitch angle helix of the present
invention is capable of achieving, in absolute terms, more gain than a
helix having a fixed pitch angle.
For this purpose, the pitch angle .alpha..sub.i in degrees at any turn i
along a helical winding of N turns, relative to the distal end thereof, is
preferably defined in accordance with equation (1) as:
##EQU1##
Such tapering of the pitch angle of the helical winding allows the use of
considerably shorter helical winding than would otherwise be possible for
the case of a constant pitch angle winding. This results in a much more
compact and lighter weight antenna array, particularly where the number of
antenna elements is relatively large (as in the case of the thirty-six
element array of the present example).
FIG. 5 is a plot of the performance of an antenna configuration of the
present invention, described above, illustrating an effective irradiated
power (EIRP) density vs. scan angle characteristic 50, that falls within
the baseline 60 currently required by INTELSAT, which exempts the array
from formal verification testing, and thereby facilitates link
implementation.
As will be appreciated from the foregoing description, mutual coupling and
grating lobe problems of conventional axial mode antenna arrays are
substantially reduced or eliminated by the relatively compact helical
antenna configuration of the present invention. Aperiodically distributing
a plurality of tapered pitch helical antenna elements around the antenna's
boresight axis, and separating the elements by a distance of at least two
wavelengths for a 17 dBiC helix not only minimizes grating lobes, but
optimizes the gain of the overall array.
While I have shown and described an embodiment in accordance with the
present invention, it is to be understood that the same is not limited
thereto but is susceptible to numerous changes and modifications as are
known to a person skilled in the art, and I therefore do not wish to be
limited to the details shown and described herein, but intend to cover all
such changes and modifications as are obvious to one of ordinary skill in
the art.
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